© 2002 Microchip Technology Inc.. DS00781A-page 1AN781 Solving Sensor Offset Problems with TC7106 INTRODUCTION Design engineers sometimes have to interface the TC7106 and similar ADCs to
Trang 1© 2002 Microchip Technology Inc DS00781A-page 1
AN781 Solving Sensor Offset Problems with TC7106
INTRODUCTION
Design engineers sometimes have to interface the TC7106 and
similar ADCs to “non-ideal” sensors A very common problem is
that the sensor often does not give a “zero” output where the design
wants a zero reading.
One example of a “non-ideal” sensor is a diode used as a
tempera-ture sensor The diode typically changes –2mV per degree Celsius,
but the change is from the diode’s forward voltage of 600mV or so.
In order for the display to read “000” at 0.0 degrees, an offset must
be provided.
The differential inputs of the TC7106 yield an easy solution to the
offset problem Figure 1 shows a simple thermometer with a diode
sensor Because the diode voltage decreases as temperature
increases, IN LO is connected to the diode temperature sensor.
The IN HI input is connected to a trimpot which is used to cancel the
diode’s forward voltage.
FIGURE 1: Positive offset.
Author: Wes Freeman,
Microchip Technology, Inc.
The offset problem gets a little more difficult, however, if a sensor requires a negative offset The easiest way, shown in Figure 2, is
to use a LM285-1.2 reference This will provide an offset of up to –1.20V The only “trick” to this circuit is that resistor R1 must source enough current for the LM285-1.2 plus a few extra microamps for the COMMON input.
Figure 2 also demonstrates the utility of the TC7106 families’ differential reference The effective reference voltage is simply the difference between the REF HI and REF LO inputs In this way, the same LM285-1.2 can be used to produce both input offset and reference voltages.
IN LO
IN HI
REF HI
REF LO
Common
TC7106
VFS = 2.0V
10k
R2
10k
R1 IN4148
Sensor
9V
Trang 2© 2002 Microchip Technology Inc DS00781A-page 2
FIGURE 2: Negative offset.
REF HI
Common
TC7106
R1 22k
LM285-1.2
9V
IN HI
IN LO
REF LO
36k
Input
+
–
.01 Offset
Adjust
Reference Adjust 50k 1M Ω
Trang 3 2002 Microchip Technology Inc DS00781A - page 3
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Trang 4DS00781A-page 4 2002 Microchip Technology Inc.
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DS00781A
W ORLDWIDE S ALES AND S ERVICE